Belinda Robson

This Waterlines report is part of a series of papers commissioned on issues relating to Australian aquatic ecosystems. These Waterlines reports will contribute to improved environmental water management by stimulating discussion, synthesising current thinking, identifying knowledge gaps, and highlighting areas that warrant further investigation.

Aspects of river flow regimes, termed flow components, may be restored with an overall aim of rehabilitating some aspects of river ecology. Flow regime restoration can involve releases from storages, or the regulation of direct water extraction, or even the reinstatement of ceaseto- flow periods in seasonally flowing rivers. Flow restoration is guided by environmental flow objectives, and a variety of these are cited in the literature. Some are highly specific ecological objectives, such as triggering fish spawning or waterbird breeding. Some are geomorphological objectives, such as restoring particular channel forms, where ecological effects are expected to cascade from this restoration of physical structure. Still others are large-scale, long-term declarations of whole-river objectives, such as the ‘Healthy Working River’ objective for the River Murray. Clearly, as these objectives vary, so will the number and types of strategies used, the spatial and temporal scaling of expected ecological outcomes, and the recovery pathways that produced these outcomes. The aim of this document was to identify and describe current work on recovery pathways after flow restoration in rivers so that knowledge gaps and directions for future investment could be identified.

We reviewed the scientific literature on recovery pathways after flow restoration in rivers and found many examples of the restoration of different types of flow components that had led to measurable ecological improvements. However, the biophysical pathways that led to these improvements had rarely been explicitly identified. That is, post-release monitoring could identify ecological outcomes, but it is often not designed to measure the ecological and physical processes that lead to these outcomes. There is a need for further research that focuses on these processes and how they are initiated and supported by flow restoration. It is also important that more of the outcomes of flow restoration studies (both monitoring and process-based studies) are published in the scientific literature where they are peer-reviewed and can be broadly accessed, in perpetuity, to enable restoration science to progress. In particular, this would assist in determining whether restoration of particular flow components results in the same recovery pathways in different places and therefore in developing generalisable flow release strategies.

The literature indicated that recovery pathways after flow restoration in rivers occur in two ways: by triggering direct biological responses in plants and animals and by initiating a sequence of ecological processes. Importantly, while flow releases directly trigger responses such as germination or spawning, the ultimate success of these ecological processes depends on other slower processes being initiated and sustained. That is, processes involving food webs and nutrient cycles, for example, are needed to support newly recruited plants and animals so that they develop into reproductive adults. It is also important to note that, when a particular flow component is restored, not all of the ecological processes depending on it will necessarily be restored because of the interacting effects of other disturbances (for example, presence of exotic species).

Different kinds of flow releases may initiate or support recovery pathways, and some options require more consideration and research in the Australian context. In particular, dam removal and the reinstatement of cease-to-flow periods have received little attention. Further innovation may also be possible, such as the strategic use of hypolimnetic releases to limit the impact of increased water temperatures resulting from climate change. Several case studies now show that recovery pathways in rivers will not be initiated or sustained if releases are too small in volume or duration, or if they are not repeated often enough. Environmental risks resulting from flow restoration are generally dealt with fairly briefly in the literature. They include: increased waterlogging, interactions with saline or sulfidic soils, blackwater events (in floodplain rivers), and reduced water quality from hypolimnetic releases.

We present an outcomes-based conceptual model, the Recovery Cascade Model, for predicting recovery pathways after restoration of components of flow regimes. The model is adaptable for restoration of different or multiple components. It aims to clarify the outcomes delivered at each stage of the process and how these outcomes can be blocked by barriers, and so fail to accurately represent the rates of ecological processes. The model presents the recovery processes as sequential and stepwise, although in reality, ecological processes occur partially in sequence and partially in parallel. A sequential model makes it easiest to evaluate outcomes and barriers to restoration. In all cases, more observations are needed of real recovery pathways after all types of flow restoration in rivers, and the model will need to be adapted to include this new information. The model presented here can be adapted for existing situations by individual managers for their own systems. Specific environmental risks, such as those related to water quality or salinisation should be incorporated on a case-bycase basis.

In conclusion, recovery pathways are described for some types of flow restoration in a few cases, but we are far from evidence-based documentation of recovery pathways. Further research investigating the biophysical processes following flow restoration is required. In planning recovery trajectories for environmental flow, there needs to be an audit and full evaluation of the interacting disturbances that could strengthen or dissipate the effects of the flow. Designs for monitoring responses to environmental flow releases are improving rapidly, but process-based studies of recovery pathways need further development. In particular, recovery pathways after the restoration of multiple flow components need to be studied and potential barriers to restoration need to be identified and consideration given to how they can be overcome.